Mechanical wrist watches employ spring wound mechanisms which convert the stored energy of the spring into mechanical movement of the watch's hands. Typically, such watches must be hand wound every two or three days to assure continuous operation. If the user forgets to wind the watch, the spring motor will eventually unwind causing the watch to cease operation.
Self-winding mechanisms are known for keeping a mechanical wrist watch wound while it is worn by a user. Most self-winding mechanisms employ a rotary pendulum mechanism for winding the spring motor. The random arm movements of the user as he or she engages in normal day-to-day activities causes the pendulum to swing back and forth. The motion of the pendulum is used to wind the spring. The spring motor stores sufficient energy to keep the watch operating overnight, whether worn or not. Thus, the daily use of the watch will be sufficient to maintain continuous operation without the need to manually wind the watch spring.
It is not uncommon for a person to own more than one watch. For example, a person may have a stainless steel watch which is used for sports events, a second watch for normal daytime use, and a third watch for evening or formal events. Consequently, there may be significant periods of time during which a particular watch is not used. Unless the owner remembers to manually wind the watches, the spring motor will eventually unwind and the watch will cease operation. The task of keeping multiple watches wound and operating is an inconvenience. For this reason, many people depend on a watch winder to keep their watches wound during periods of non-use.